This invention pertains to the field of ionizing gas samples for mass spectrometers, and in particular to using one electron beam traversing two ionization regions to provide separate streams of ions for independently measuring both total pressure and partial pressure of the gas sample.
Many scientific instruments, such as mass spectrometers, require generation of ions so that the ions may be accelerated or otherwise input into the instrument for sample identification, measurement, and other purposes. For a quadrupole residual gas analyzer, it is desirable to indicate the ionization current as a total pressure measurement, in addition to filtering the ion current to indicate specific ion species.
A conventional ion source includes a filament acting as an electron emitter, an ion volume containing rarified gas, and an ion accelerator. Electrons from the filament enter the ion volume through an opening in an ionization chamber surrounding the ion volume to ionize gas molecules within the ion volume. The ion accelerator draws the resulting ions out of the ion volume and focuses them into a beam of ions suitable for injection into the quadrupole filter or other mass analysis instrument.
When using such a device, it is usually desirable to have an accurate measurement of the ion stream or ion current being supplied to the quadrupole mass filter or other instrument. One conventional method for measuring ion current is typically to measure an ion current at the ion accelerator, since a portion of the ion stream impacts on the ion accelerator. However, this method has several drawbacks. The principal drawback is a change in ion fraction impacting acceleration as the mass range is scanned. For example, the ion accelerator often has electrical leakage. The measurement may also be affected by stray currents from the ionization process.
Another conventional method is to place an ion collector in the path of the ion stream. However, this method has the drawback of interfering with the ion stream. In addition, in both of the above methods, and in others where only a fraction of the ion stream is measured, it is difficult to judge the exact useable ion current by measuring the xe2x80x9ctestxe2x80x9d fraction, because as the intensity of the total ion stream varies, the ratio between the xe2x80x9cutilizablexe2x80x9d portion of the ion stream and the xe2x80x9ctestxe2x80x9d portion striking the measurement collector may vary in unknown ways.
Briefly stated, an apparatus for determining both total and partial pressures of a gas using one common electron beam includes a partial pressure ionization region and a total pressure ionization region separated by a grid or aperture. A filament produces a plurality of electrons which are focused into an electron beam by a repeller and an anode. The interaction between the electron beam and molecules of said gas within the partial pressure and total pressure regions produces first and second ion streams. A focus plate is biased such that the first ion stream is directed to an analyzer which calculates the partial pressure of the gas. An ion collector collects the ions from the second ion stream, where the resulting reference current is used to determine the total pressure of the gas.
According to an embodiment of the invention, an apparatus for determining a total pressure of a gas includes an ionization chamber; the ionization chamber having first and second ionization regions, wherein a boundary between the regions is defined by an anode grid or aperture; means for producing an electron beam passing through the first and second ionization regions, whereby an interaction between the electron beam and molecules of the gas within the ionization chamber produce first and second ion streams from a same gas density; means for directing the first ion stream to an analyzer; and means for directing the second ion stream to an ion collector.
According to an embodiment of the invention, an apparatus for determining a total pressure of a gas includes an ionization chamber; the ionization chamber having first and second ionization regions, wherein a boundary between the regions is defined by an anode grid or aperture; a filament for producing a plurality of electrons; a repeller; the repeller and anode operatively associated to focus the plurality of electrons into an electron beam passing through the first and second ionization regions, whereby an interaction between the electron beam and molecules of the gas within the ionization chamber produce first and second ion streams from a same gas density; a focus plate biased such that the first ion stream is directed to an analyzer, wherein the analyzer includes means for calculating at least one partial pressure of the gas within the ionization chamber; an ion collector biased such that the second ion stream is directed to an ion collector; means for measuring a reference current produced by the second ion stream at the ion collector; and means, using the reference current, for calculating the total pressure of the gas within the ionization chamber.
According to an embodiment of the invention, a method for determining a total pressure of a gas includes (a) providing an ionization chamber, the ionization chamber having first and second ionization regions, wherein a boundary between the regions is defined by an anode grid or aperture; (b) producing an electron beam passing through the first and second ionization regions, whereby an interaction between the electron beam and molecules of the gas within the ionization chamber produce first and second ion streams from a same gas density; (c) directing the first ion stream to an analyzer; and (d) directing the second ion stream to an ion collector.